Bidirectional inverters are capable of transferring energy both from the DC side to the AC side and from the AC side to the DC side. An example of this requirement is the provision of reactive power on the AC side of the inverter. In this case the inverter only has to transfer energy from the DC side to the AC side during part of the grid period, whereas for the rest of the time the energy flow reverses.
For inverters that use so called IGBT components as semiconductor switches, this requirement generally does not pose a problem if a diode is connected antiparallel to each IGBT. When the energy flow reverses, current flows through the diode instead of the IGBT. However, if other semiconductor switches such as a MOSFET are used, with internal, parasitic diodes, problems often occur with the internal, parasitic diodes in these components. These diodes are usually very poorly suited to the switching operation as they have a high reverse recovery charge and a hard cut-off of the current when commutating. However, it cannot be avoided that when the energy flow reverses, these diodes take over the current just like the diodes that are specifically fitted antiparallel to the IGBTs. The internal, parasitic diodes then may cause switching loss as a result of their recovered charge and may provide poor EMC properties as a result of the hard cut-off of the current. Special circuit arrangements therefore may have to be used if bidirectional inverters are built using MOSFET or comparable semiconductors as switches.
One such circuit is known from U.S. Pat. No. 6,847,196. Here a DC/AC converter with four switches and current paths separated by additional, saturable chokes is used. Four additional freewheel diodes are present. This arrangement reduces or prevents current flowing through the antiparallel diodes that are present internally in the switches. However, a disadvantage in this circuit is that decoupling between the DC source and the circuit arrangement does not occur in any switching state. Therefore, as with any conventional full bridge, in the event of a switching operation in only one half of the bridge, a voltage discontinuity occurs between the input side and output side. If the output side of the inverter is connected to the public grid and therefore grounded, the input side potential changes in a step-like manner compared to the ground potential. This is disadvantageous for photovoltaic generators as a source, for example, as due to their design they have a high capacitance with regard to the enclosure, with the result that undesired capacitive parasitic currents and/or dangerous contact voltages can occur.
High-frequency potential discontinuities can only be avoided with this circuit if both halves of the bridge are switched simultaneously, which however reduces the efficiency of the circuit. Alternatively, a transformer could be connected downstream of the circuit on the grid side. But in this case the overall efficiency is reduced as well.
The German patent specification DE 102004030912 describes a photovoltaic inverter that comprises four bridge switches and an additional decoupling switch outside of the bridge and in the DC circuit. This circuit solves the problem of the high-frequency parasitic currents with reasonable efficiency. During the freewheel phases, a freewheel path, in which the freewheel current flows, is separated from the photovoltaic generator by the decoupling switch. As a result, the potential discontinuities and charging of the parasitic capacitance between generator and ground is avoided, so that corresponding high-frequency parasitic currents are diminished.
The disadvantage of this circuit is that with bidirectional operation optimum components cannot be used for the switches and diodes. Due to the different energy flow directions, antiparallel diodes must be provided for all semiconductor switches. If MOSFET or comparable transistors are used as switches, however, their internal, parasitic diodes still take over some of the current with the negative effects already described.
Another solution to the problem of high-frequency parasitic currents is described in EP 1369985. Here a conventional full bridge with four bridge switches is complemented by two switchable connecting paths between the bridge outputs, with the result that likewise a freewheel path with floating potential that can be decoupled from the DC side is created. Here too, however, at least for the bridge semiconductors, antiparallel diodes have to be provided, with the result that certain semiconductor types like MOSFET cannot be used without problems.
It is therefore desirable to find a simplified, bidirectional, and transformerless inverter topology, in particular for the use with photovoltaic generators, which avoids potential discontinuities between the AC side and the DC side while providing a high conversion efficiency.